U.S. Pat. No. 4,315,263 of applicant Neidell, which reissued as U.S. Pat. No. Re. 31,509, describes an echo location system where phase is encoded into a wavefield according to the direction of transmission. This allows solution for the direction of travel path from the signal phase, so that one may echo locate with a single source and receiver. The time of signal travel gives the radial distance to the target, and the phase gives angular co-ordinates.
In marine navigation or towed marine streamer positioning, triangulation is often used to solve for a position. In extreme weather conditions, a partial loss of signals results in ambiguous solutions. In marine streamer location using acoustic methods, bubbles from an air-gun seismic source can introduce serious uncertainties in range values (see Ha, SEG 60th Annual International Meeting Expanded Abstract, pp. 878-880, 1990). Should the Neidell method be incorporated, one would gain additional information by knowing the direction of transmission. This extra data could very well compensate for the loss of other data thus increasing the reliability of a navigation or positioning system with many benefits depending on the particular application.
The Neidell method also allows one to image with a coincident source and receiver and is suitable where space is limited or where transducers can only be placed at certain locations. Such limitations can be found for example in a wellbore and this method becomes advantageous should one consider the task of imaging ahead and around while drilling through subsurface formations. Acoustic imaging while drilling would have important contributions: one being control of drilling direction, and another evaluation of formations not yet encountered.
In horizontal drilling, for example, one must avoid sudden bends in the drilling path if the bedding dip changes (see Martin in The Leading Edge, Vol. 11, No. 8, pp. 21-26, 1992). Therefore, adjustments in the drilling trajectory need to be made well in advance to anticipate changes of dip. Acoustic reflections from rocks reveal different physical properties from those obtained using electromagnetic and radiation methods of measurement. Acoustic reflections thus contribute further information about formations not yet penetrated. Additionally, hazards to the drilling may be detected well in advance so proper precautions may be applied.
Although the Neidell method was proposed a decade ago, so far as is known it has not been yet implemented as a commercial product. One problem has been the generation of signals whose phase can be controlled to change with direction.
Echoes received from small reflectors have been found to be significantly distorted in phase compared to the original impinging wavefield. Similarly, certain propagation media can cause phase distortion in the signals in transit. Such distortions would normally cause a problem if a phase-encoded wavefield is utilized for direction finding with a single source and receiver.